Shift transmissions, in which a medium conducting an operating pressure can alternately be applied to clutches, brakes, a hydrodynamic clutch, and/or a hydrodynamic converter, to vary the transmission ratio of the speed and/or the torque between a transmission input shaft and a transmission output shaft by opening and closing the clutches and/or brakes and/or by hydrodynamic power transfer using the hydrodynamic clutch and/or the hydrodynamic converter, and control devices for setting the operating pressure are described, for example, in the following documents:
DE 35 42 956 C2
DE 36 30 792 C2
DE 43 33 899 A1
DE 699 28 730 T2
DE 103 58 005 B3
DE 196 34 761 B4
DE 37 29 463 C2
DE 36 30 768 C2.
The methods according to which these control devices operate provide a continuous variation of the operating pressure as a function of different variables, for example, the engaged gear, the power of a drive engine, which drives the transmission, as a function of a shift action or on the basis of the differentiation between the traction operation and the overrun operation of the drive engine. For example, the control pressure which acts on the operating pressure is set proportionally to the engine load.
Through the known methods for continuous variation of the operating pressure, unnecessary power consumption of the pump, typically an oil pump, which delivers the medium conducting the operating pressure, in particular oil, to bring it to the desired pressure, can be avoided, in that only as much operating pressure is generated as is considered to be sufficient on the basis of the mentioned dependencies. However, the comparatively large outlay is problematic with such a continuous setting of the operating pressure. For example, pumps which can be regulated or controlled must typically be provided.
A further disadvantage of the known method is that, on the one hand, because of manufacturing tolerances and, on the other hand, because of wear over the operating time, the actually resulting operating pressure can deviate from the selected theoretical specifications and dependencies. This can have the result, in particular with increasing age of the transmission or the oil used in the transmission, that an undesired reduction of the operating pressure occurs, which can result in control errors in the transmission and increased wear of the components participating in shift actions, in particular the clutches and brakes.